1. Field of the Invention
This invention relates to packaging for integrated-circuit chips and, more particularly, to packaging techniques which provide improved cooling for an integrated-circuit chip.
2. Prior Art
Efficient removal of heat from a packaged integrated-circuit chip is very important because the temperature of the integrated-circuit chip must be kept below certain designated temperatures to avoid degraded performance or to prevent damage to the chip. Very often, a silicon integrated-circuit chip is enclosed in a package, where the package itself has a significant thermal resistance. Heat energy travels in series from its source in the integrated-circuit through the thermal resistance of the package to the ambient. For a given amount of heat power being generated by the integrated-circuit chip, higher values of thermal resistance produce higher temperatures in the integrated-circuit chip. The overall thermal resistance for the series combination of the chip and the package is always greater than the thermal resistance of the package alone so that the thermal resistance of the package often limits the thermal performance of the integrated-circuit chip.
Two commonly used packages for integrated-circuit chips are a pin-grid-array (PGA) package and a plastic quad flat pack (PQFP) package configurations. The PGA package is square and has a number of downwardly extending, uniformly-spaced terminal pins arranged on a grid. A PQFP package has bent, flat leads extending from all four sides of the package. Both types of package structures enclose a silicon chip and have thermal resistance, preventing direct removal of heat from the integrated-circuit chip. No matter how well the ambient removes heat from the package, the package still remains as an insulator. It is not possible to achieve a thermal resistance lower than the thermal resistance of the package.
FIG. 1 shows a standard pin-grid-array PGA package assembly 10. The assembly includes a package body 12 formed of a ceramic or molded plastic material. A recessed cavity 14 is formed in a front side of the package body 12 for receiving an integrated circuit die, or chip, 16. The integrated-circuit die 16 is fixed with a layer 18 of die-attach material to the interior surface of a rear wall 20 of the package body 14, where the rear wall 20 defines the interior boundary of the cavity 14. Wire-bonding pads on the surface of the integrated-circuit die 16 are connected by bonding wires (typically shown as 22, 24) to corresponding bonding-pad portions of internal conductors (typically shown as 28) contained within the package body 14. The internal conductors 28 are connected with conventional feed-through solder connection means to respective connection pins (typically shown as 30, 32). A problem with this conventional PGA package configuration is that heat from the integrated-circuit die 18 must travel through the thickness of the wall 20 and other parts of the body of the package to escape to the ambient. Consequently, the thermal performance of the package is limited by the thermal coefficient of the wall material and the thickness of the wall 20.
A number of prior solutions are available for lowering the thermal resistance of a standard PGA package configuration for an integrated-circuit chip. One solution is to use a package material which has a better thermal conductivity. For example, a ceramic material can be used instead of a plastic material for the packaging material.
FIG. 2 shows another technique for improving the thermal performance of a pin-grid-array PGA package assembly 50. The pin-grid-array PGA package assembly 50 is similar to the package assembly of FIG. 1 and the same reference numerals are used for like elements. The pin-grid-array PGA package assembly 50 uses an additional component within a package body 52 for the package assembly 50. That component is an internal heat spreader 54, which is a plate, or slug, formed of copper or copper/tungsten material. The integrated-circuit chip 16 is bonded to the heat spreader 54 with a layer 56 of suitable die-attach material, as indicated in the figure.
Use of the heat spreader 54 improves the heat transfer characteristics of the package 50, but the heat spreader 54 is still in the series thermal path from the ambient to the integrated-circuit chip 16. A significant problem with using a heat spreader 54 is that cracks develop in the body of the package 52 near the junction of the heat spreader 54 and the package 52. As the metal of the heat spreader 54 expands and contracts with changes in temperature, the stronger, more rigid metal heat spreader exerts stress on the package body 52 and also on the integrated-circuit die 16, causing the package material to crack. To reduce the amount of stress on the package body, the heat spreader 54 is kept small, that is, approximately the same size as the area of the integrated-circuit die 16.
FIG. 3 shows another type of conventional package assembly for integrated-circuits. A standard plastic quad flat-pack package PQFP 100 includes a package body 102 formed of molded plastic material. The molded-plastic body 102 is molded around an integrated-circuit die 104, which is fixed to a die-attach pad 106 portion of a standard leadframe, using a suitable layer of die-attach material 108, as indicated in the figure. Wire-bonding pads on the surface of the integrated-circuit die 104 are connected by bonding wires (typically shown as 110, 112) to corresponding bonding finger portions of leads (typically shown as 114, 116), which extend out of the package body 102, as indicated in the figure.
Similar to the case of the standard PGA package, a problem with the conventional PQFP package configuration is that heat from the integrated-circuit die 104 must travel through the body 102 of the package and through the thickness of the wails of the package body to escape to the ambient. Depending on the thermal coefficient of the wall material and the thickness of the wall, the thermal performance of the package is limited by the thermal resistance of the plastic package-molding material.
FIG. 4 shows a technique for improving the thermal performance of a plastic quad flat pack PQFP package assembly 150, which is similar to the package assembly of FIG. 3 and where the same reference numerals are used for like elements. A PQFP package body 152 uses an internal heat spreader 154, which is a plate, or slug, formed of copper or copper/tungsten material. The outside surface of the die-attach pad 106 is bonded to the heat spreader 154, as indicated in the figure. Use of the heat spreader 154 improves the heat transfer characteristics of the PQFP package assembly 150.
As in the case of the PGA package, the heat spreader 154 for the PQFP package is still in the series thermal path from the ambient to the integrated-circuit chip 104. Cracks also develop in the body of the package 152 near the junction of the heat spreader 154 and the package 152. As the metal of the heat spreader 154 expands and contracts with changes in temperature, the stronger, more rigid metal heat spreader 154 exerts stress on the package body 152 and on the integrated-circuit die 104, causing the package material to crack. To reduce the amount of stress on the package body, the heat spreader 154 is kept small, approximately the same size as the area of the integrated-circuit die 104.